Electron radiographic imaging chamber with current enhancement

ABSTRACT

An improved imaging chamber for an electron radiographic system with a d.c. potential and an a.c. potential in the radio frequency range connected across the electrodes to provide increased current collection while reducing interelectrode discharge problems.

United States Patent 1191 Proudian [451 Feb. 12, 1974 [54] ELECTRONRADIOGRAPHIC IMAGING 3,443,097 5/l969 Smith 250/382 X CHAMBER WITHCURRENT ENHANCEMENT Primary Examiner-James W. Lawrence [75] inventor:Andrew P. Proudian, Chatsworth, Assistant Examiner-Davis L. WillisCalif. Attorney, Agent, or FirmHarris, Kern, Wallen & [73] Assignee:Xonics, Inc., Van Nuys, Calif. Tmsley [22] Filed: Nov. 10, 1972 [21]Appl. No.: 305,339 [57] ABSTRACT An improved imaging chamber for anelectron radio- 250/ graphic system with a dc. potential and an a.c.potential inrthe radio frequency range connected across the [58] new of250/315 315/176 electrodes to provide increased current collectionReerences Cited while reducing interelectrode discharge problems.

UNITED STATES PATENTS 4 Claims, 1 Drawing Figure 3,594,162 7/1971 Simmet al 250/315 X 25 5 SUPPLY I OSCILLATOR PATENIEB FEB I 21974 SUPPLYOSC/LL/QTOR .E EcTRoN RADIOGRAPHIC IMAGING CHAMBER WITH CURRENTENHANCEMENT This invention relates to the creation of X-ray imageswithout the use of conventional X-ray film, sometimes referred to asionography. In particular, it relates to the technique described in thecopending application of Muntz, et al, Ser. No. 26l ,927, filed June I2,1972, entitled Radiographic Systems with Xerographic Printing, andassigned to the same assignee as the present application. In such asystem, an X-ray opaque gas at high pressure (e.g. 2O atmospheres) isused between two electrodes in an imaging chamber to produce aphotoelec'tric current within that chamber as a function of X- raysentering the chamber. The current is collected on a dielectric sheetplaced on one or the other of the electrodes, resulting in a latentelectrostatic image on the sheet. The latent image is then made visibleby xero graphic techniques Collection of the primary photo electronscreated by the X-rays absorbed in the interelectrode gas filled gap andof the secondary electrons created by collisions of the primaries withthe gas atoms is achieved by use of an accelerating potentialdifference, typically of the order of 5,000 volts, applied between theelectrodes.

The value of the applied d.c. potential between the anode and cathode isselected in the above system so as to satisfy two constraints. On theone hand, the applied accelerating field or potential should be as highas possible so as to maximize the current or total electron countproduced per absorbed X-ray quantum. The higher the collected charge,the more readily the latent electrostatic image can be made visible,e.g. by powder cloud development, because the fields controlling tonerparticle deposition are clearly proportional to charge on the receptor,and at very weak fields other forces on the toners (e.g. aerodynamicforces) and random momentum of toner particles lead to random tonerdeposition and therefore poor imaging. In addition, at higher chargelevels the effects of stray charge collected on the receptor (bothbefore and after the exposure), which acts as a source of noise, and isthe limiting factor in contrast resolution, becomes less important.Also, at very weak fields the rate of toner deposition for toner that iscontrolled by the imaging fields becomes too slow for practical use ofthe system. By these criteria one would even want to operate the devicein the so-called electron avalanche region of the Townsend dischargecurve, in which significant current gains over the primary photoelectriccurrent are possible. However, as the accelerating potential isincreased, the operation of the ionization chamber becomes increasinglyunstable, leading in particular to localized discharges, generallyaggravated by the presence of small foreign particles (dirt, lint and soforth) within the imaging chamber. Indeed, unless particular care istaken to keep the chamber free of such foreign particles dischargesandattendant discharge spots in the resultant image occur at values ofthe accelerating potential well short of full collection potential V,.,(where V may be defined as that value of the potential at whichsubstantially all the primary and secondary electrons created byphotoelectric absorption of the X-rays are collected at the receptor).More correctly, the controlling quantity, which determines the collectedcurrent, is the accelerating field divided by the pressure p, i.e., thequantity E/p, rather than the voltage V (E/p) pd,

where d is the gap width. Current collection in the imaging chamber isdiscussed more fully below.

Early onset of discharge spots due to foreign particles results from thefield concentrations and effective reductions in electrode spacing whichoccur when a particle of dirt, or of lint, or other substance is placedin the interelectrode gap. Such particles therefore reduce the value ofthe applied potential at which localized breakdown would otherwiseoccur. Typically, the po' tential at which discharge .spots appearisreduced from 5,000 Volts to 3,000 or 4,000 V, for typical operatingconditions of the imaging chamber, in going from a rel atively clean toa relatively dirty chamber. When great pains are taken to maintainchamber cleanliness, discharge free operation is possible at voltages upto 7,000 to 8,000 volts, which is typically the full collectionpotential. In operation at 3,000 V, the current collected can be halfthe full collection current, at high X-ray energies.

Experimentally, it is found that the local electron avalanche leading todischarges builds up in times of the order of a few up to a few hundredmicroseconds, whereas X-ray exposure durations in medical radiology lastanywhere from milliseconds to several seconds. There is therefore no wayto avoid such discharges by restricting the time during which theaccelerating potential is applied, since the time during which it mustbe on is much longer than the time required for the discharge todevelop.

Thus, in the prior art devices, one must compromise, in choosing theoperating value of the accelerating potential, between maximizing thecollected charge for a given exposure level, which leads as explainedabove to a more easily developed and less noisy visible image, andkeeping the accelerating fields which collect the current to low enoughvalues so as to avoid discharge spots which constitute diagnosticallydamaging image deletions, without having to go to extreme lengths toprevent any foreign matter from entering the imaging chamber.

Accordingly, it is an object of the present invention to provide a meansfor increasing the current and collected charge on the insulatingreceptor of the electronradiographic imaging chamber resulting from anabsorbed X-ray quantum without resulting in unstable operation andlocalized discharges and discharge spots. It is also an object of theinvention to make the performance of the imaging chamber less sensitiveto dirt, and therefore to enhance the practical usefulness of electronradiography.

This new and improved performance is achieved by augmenting theappliedsteady or do. accelerating or current collecting electric fieldin the imaging chamber with an a.c. field which does not itself collectcurrent but does increase the number of electrons collected by the dc.field, at any given value of the latter, above the number that wouldresult in the absence of the a.c. field.

Other objects, advantages, features and results will more fully appearin the course of the following description. The single figure of thedrawing illustrates an electron radiographic system with imaging chamberand incorporating the presently preferred embodiment of the invention.

X-rays are directed from a source 10 past the object 11 being X-rayed tothe imaging chamber 12, which may be conventional in design such as setout in the aforementioned copending application.

A typical imaging chamber includes a housing 13 carrying a cathode 14 onan insulator 15, with an anode 16 carried on another insulator 17. Thedielectric sheet receptor 18 may be carried on the anode, with gasintroduced into the chamber at 19 filling the gap between theelectrodes.

A d.c. field is produced across the gap by a d.c. supply connected atterminal 25 and coupled to the anode 16 through an RF choke coil 26 anda coupling inductance 27, with the cathode 14 connected to systemground.

The a.c. field is provided by a radio frequency oscillator 30 connectedto the grid of a power amplifier triode 31, with the amplifier energizedfrom the B+ supply I and with the RF output coupled through theinductance 27 to the anode 16, with a variable capacitor 32 connectedacross the triode for RF tuning.

The basis for the enchancementachieved with the system of the drawingcan be understood from the following qualitive description of thephysical processes occurring in the imaging chamber 12. Consider theevents following the absorption of a photon by photoe lectric absorptionin the K or L shell of one of the Xenon atoms in the gas gap. Theresulting ejected electron, which has an energy typically of a few tensof kilovolts, produces a trail or swarm of secondary electrons by impactionization of other Xenon atoms, which lose electrons from their outershells. The swarm of secondary electrons, which are rapidly thermalized,then drifts towards the collecting electrode under the influence of theapplied field. The drift velocity of the electrons is approximatelyproportional to the applied field. In their drift, the electrons gainenergy from the field, and lose energy in elastic and inelasticcollisions with the gas molecules. The electrons can also recombine withthe ions present in the gas, either directly or more frequently byattachment to a neutral molecule followed by negative ion-positive ionrecombination. Re-

combination is the only effective loss mechanism of the secondaryelectrons under the high pressure conditions of operation of the imagingchamber.

At low values of the applied field, the electron (and negative ion)drift velocity .is low, and the energy gained by the electrons from thefield per unit length of travel in the direction of the field is low, sothat a majority of the secondary electrons produced in the swarm arelost by recombination before they reach the collecting electrode. As theapplied potential is increased, the electron drift velocity increaseswhile the probability of recombination (and attachment), which isstrongly dependent on electron energy, decreases because of theincreased energy gained from the field by the electrons betweencollisions. Thus the collected current increases, until there comes apoint, at what we have called full collection potential V,, whenessentially all the secondary electrons are collected, and a so-calledplateau is reached in the current voltage characteristic, where furtherincreases in accelerating potential do not result in increased current.

The above description is an oversimplification in that it omits anotherprocess which occurs in the gas, namely the increase in the number ofelectrons in a swarm due to inelastic collisions of the electrons withneutral molecules in the gas, which result in the creation of additionalion pairs. In fact, the ionization and recombination are competingprocesses, and the former increases with applied field (and thereforefield strength)just as the latter decreases with increasing appliedfield. Ionization becomes significant well before recombination isreduced essentially to zero, and indeed there is no true plateau incollected current, but rather a very slow increase with increasdvoltage.

Beyond a certain value of the applied potential, the rate of ionizationexceeds the rate of recombination and there is a net gain in collectedcurrent. This is the so-called avalanche regime in which each initialsecondary electron leads to collection of more than one electron at thereceptor.

As the applied voltage is increased into the avalanche region, secondaryprocesses begin to appear which result in creation of electrons by theaction of photons created by the collision processes and by othereffects, which lead to more avalanching, followed by further creation ofelectrons and so forth, so that the discharge becomes effectively selfsustaining. This is the phenomenon of breakdown, occurring at thebreakdown value V, of applied potential. Since the electron avalancheand secondary creation processes are random in nature, isolatedbreakdowns will occur randomly in localized regions of the chamberbefore the point of full breakdown of the gas in the gap has beenreached. When foreign particles are present, they will tend to increasethe field strenghts in their vicinity, and therefore lead to the sameeffect as if the applied field had been increased, namely localizedbreakdown, resulting in discharge spots in the developed image. Thesecan be avoided by reducing the applied field to the point where even thelocal enhanced fields are not sufficient to produce breakdown. However,as this is done, one suffers a loss in collected current, because of thedirect or indirect recombination losses described above.

It has been found that the recombination losses can be eliminated orreduced by increasing the average electron energy without increasing theapplied accelerating field, since recombination rates fall off verysharply with electron energy. This can be done by radio frequency energyto pump energy into the free electron gas in the chamber. Sucha'procedure will be very effective in permitting full currentcollection, since an increase in mean electron energy to only I electronvolt, requiring typically a power dissiptation of 10' watts/cm ofimaging chamber, or about 10 watts total for a typical chamber imagearea, assuming a modest pumping efficiency of 10", will reduce therecombination rate by one to two orders of magnitude. The use of highradio frequency fields in the 10 MHz to MHz region appears to be themost practical and efficient. If frequencies significantly below 1 MHzwere used, the applied a.c. field would act much as an addition to thesteady field, with respect to breakdowns, since the formation ofbreakdown avalanches have characteristic times of the order ofmicroseconds. On the other hand, at frequencies above I0 Hz, whichcorresponds to a wavelength of 30 cm, field nonuniformities over thedimensions of the imaging chamber would become significant. At laserfrequencies, coupling of power to the electron gas would become quiteinefficient, and in any event there is nothing to be gained by going tosuch high frequencies. Indeed, the criteria for selecting frequenciesare, at the lower end, that the imposed fields should not appear as d.c.fields with respect to the breakdown phenomenon, and at the upper endthat uniform enhancement, and therefore uniform fields, and effectivecoupling to the free electron gas be achieved.

These criteria lead to the choice of the MHZ to 100 MHz frequency range.The effective a.c. field strength E required for an applied a.c. field Eis simply achieved. Note that by use of ac; fields, itbecomes possibleto operate the imaging chamber somewhat into the avalanche region (i.e.,to induce electron multiplication in the ac. field) and thereby achievegains in current beyond the full collection current. However, the ac.field must be kept to values below those at which it would itself inducegas breakdown. It is interesting that the threshold for ac. breakdownwill in fact be raised by the presence of the dc. field which inhibitslocal electron breakdown. Thus the do. and ac. fields aid each other interms of permitting full current collection and possibly amplificationwithout breakdown. Typical values for the tandem fields would be, interms of the applied potential differences around 3,000 volts for thedc. potential, and around 3,000 volts (r.m.s.) for the ac. potential.The dc. potential may be reduced to as low as 1,000 volts, or

be as high as 10,000 volts for large gas gaps 1cm), while the ac.potentials may also range as low as 1,000 volts and as high as10,000volts.

I claim:

1. In an electron radiographic system, the combination of:

an imaging chamber having spaced electrodes with a gas filled gap therebetween;

a means for connecting a dc. potential and an ac.

potential across said electrodes to produce an electric field in saidgap having a steady state collecting component and an oscillatingionizing component.

2. A system as defined in claim 1 including means for generating saida.c. potential in the range of about 10 Hz to about 10 Hz.

3. A system as defined in claim 1 including means for generating saida.c. potential in the range of about 10 Hz to about 10 Hz.

4. A system as defined in claim 1 including:

a dc. power supply;

a radio frequency oscillator; and

a radio frequency amplifier energized from said do.

pp y;

with said oscillator connected as an input to said amplifier and withsaid amplifier output connected to said electrodes providing said d.c.and ac. potentials.

1. In an electron radiographic system, the combination of: an imagingchamber having spaced electrodes with a gas filled gap there between; ameans for connecting a d.c. potential and an a.c. potential across saidelectrodes to produce an electric field in said gap having a steadystate collecting component and an oscillating ionizing component.
 2. Asystem as defined in claim 1 including means for generating said a.c.potential in the range of about 106 Hz to about 109 Hz.
 3. A system asdefined in claim 1 including means for generating said a.c. potential inthe range of about 107 Hz to about 108 Hz.
 4. A system as defined inclaim 1 including: a d.c. power supply; a radio frequency oscillator;and a radio frequency amplifier energized from said d.c. supply; withsaid oscillator connected as an input to said amplifier and with saidamplifier output connected to said electrodes providing said d.c. anda.c. potentials.